WO2024071213A1 - Refrigeration cycle device - Google Patents

Abstract

This refrigeration cycle device (1) comprises a first circuit (5a), a second circuit (10), a cascade heat exchanger (35), and a control unit (80). A first refrigerant circulates through the first circuit (5a). A carbon dioxide refrigerant and a refrigeration machine oil circulate through the second circuit (10). The cascade heat exchanger (35) heats the carbon dioxide refrigerant with the first refrigerant. The second circuit (10) has a second compressor (21) and a container. The container is provided on the intake side of the second compressor (21). The container stores the carbon dioxide refrigerant and the refrigeration machine oil. The control unit (80) controls the operation of the first circuit (5a) so that the pressures or temperatures of the carbon dioxide refrigerant and the refrigeration machine oil inside the container are at least a predetermined pressure or a predetermined temperature which corresponds to a boundary temperature at which the density of the carbon dioxide refrigerant and the density of the refrigeration machine oil in the container become the same.

Description

Refrigeration Cycle Equipment

　Regarding refrigeration cycle equipment.

Patent Document 1 (JP Patent No. 5425221) discloses a refrigeration cycle device including a refrigerant circuit in which a refrigerant circulates, connecting an outdoor unit and an indoor unit with gas piping and liquid piping. Carbon dioxide (CO ₂ ) is sealed inside the refrigerant circuit as a refrigerant, and polyalkylene glycol (PAG) oil that is incompatible with carbon dioxide is sealed inside the refrigerant circuit as a refrigerant.

As disclosed in the above-mentioned Patent Document 1, in a refrigeration cycle device that uses carbon dioxide refrigerant, if a refrigeration oil that is incompatible with the carbon dioxide refrigerant is used, the density of the refrigeration oil will be smaller than the density of the refrigerant in the low temperature range of the carbon dioxide refrigerant.

The refrigeration cycle device of the first aspect includes a first circuit, a second circuit, a cascade heat exchanger, and a control unit. A first refrigerant circulates in the first circuit. A carbon dioxide refrigerant and a refrigeration oil circulate in the second circuit. The cascade heat exchanger heats the carbon dioxide refrigerant with the first refrigerant. The second circuit has a second compressor and a container. The container is provided on the suction side of the second compressor. The container stores the carbon dioxide refrigerant and the refrigeration oil. The control unit controls the operation of the first circuit so that the temperature or pressure of the carbon dioxide refrigerant and the refrigeration oil in the container is equal to or higher than a predetermined temperature or pressure corresponding to a boundary temperature at which the density of the carbon dioxide refrigerant and the density of the refrigeration oil in the container become equal.

In the refrigeration cycle device of the first aspect, the carbon dioxide refrigerant is not heat exchanged with outside air, the temperature of which naturally changes due to weather changes, but is instead heat exchanged with the first refrigerant of the first circuit in the cascade heat exchanger. Then, when the carbon dioxide refrigerant of the second circuit is heated in the cascade heat exchanger, the control unit controls the operation of the first circuit, so that even when the outside air temperature is low, the temperature or pressure of the carbon dioxide refrigerant and refrigeration oil in the container can be equal to or higher than a predetermined temperature or pressure that corresponds to the boundary temperature at which the density of the carbon dioxide refrigerant and the density of the refrigeration oil in the container become equal. Therefore, it is possible to prevent the density of the refrigeration oil in the container from becoming lower than the density of the refrigerant.

The refrigeration cycle device of the second aspect is the refrigeration cycle device of the first aspect, in which the first circuit has a first compressor. The control unit controls the rotation speed of the first compressor of the first circuit so that the temperature or pressure of the carbon dioxide refrigerant and refrigeration oil in the container is equal to or higher than a predetermined temperature or pressure.

In the refrigeration cycle device of the second aspect, the control unit controls the rotation speed of the first compressor of the first circuit so that the temperature or pressure of the carbon dioxide refrigerant and refrigeration oil in the container is equal to or higher than a predetermined temperature or pressure. This makes it easy to prevent the density of the refrigeration oil in the container from becoming lower than the density of the refrigerant.

The refrigeration cycle device of the third aspect is the refrigeration cycle device of the first or second aspect, in which the control unit switches between an operation in which the carbon dioxide refrigerant is heated by the first refrigerant and an operation in which the carbon dioxide refrigerant is cooled by the first refrigerant in the cascade heat exchanger.

In the refrigeration cycle device of the third aspect, the cascade heat exchanger is configured to be switchable between heating and cooling of the carbon dioxide refrigerant. Therefore, the refrigeration cycle device is capable of heating and cooling operations.

The refrigeration cycle device of the fourth aspect is the refrigeration cycle device of any one of the first aspect to the third aspect, and the second circuit further has a sensor. The sensor measures at least one of the temperature and pressure of the carbon dioxide refrigerant and the refrigeration oil on the suction side of the second compressor.

In the refrigeration cycle device of the fourth aspect, at least one of the temperature and pressure of the carbon dioxide refrigerant and the refrigeration oil on the suction side of the second compressor can be measured. Therefore, the density of the refrigeration oil sucked into the second compressor can be grasped.

The refrigeration cycle device of the fifth aspect is any one of the refrigeration cycle devices of the first aspect to the fourth aspect, and the second circuit further has a suction pipe, an oil return passage, and a valve. The suction pipe connects the suction side of the second compressor to the container. The oil return passage returns refrigeration oil from the bottom of the container to the suction pipe. The valve is provided in the oil return passage. The control unit changes the opening degree of the valve based on the degree of superheat of the carbon dioxide refrigerant discharged from the second compressor.

In the refrigeration cycle device of the fifth aspect, when the control unit determines that the degree of superheat (discharge superheat) of the carbon dioxide refrigerant discharged from the second compressor is low and wet suction is occurring, it can reduce the opening of the valve in the oil return passage. This makes it possible to prevent liquid-phase carbon dioxide refrigerant from flowing into the second compressor.

The refrigeration cycle device of the sixth aspect is any one of the refrigeration cycle devices of the first aspect to the fifth aspect, in which the first refrigerant includes R32, R454C, propane, R1234yf, R1234ze, or ammonia.

In the refrigeration cycle device of the sixth aspect, a first refrigerant containing R32, R454C, propane, R1234yf, R1234ze or ammonia circulates in the first circuit, allowing efficient heat exchange with the carbon dioxide refrigerant in the cascade heat exchanger.

FIG. 1 is a schematic diagram of a refrigeration cycle device. FIG. 2 is a schematic functional block diagram of the refrigeration cycle device. FIG. 4 is a diagram showing the operation (flow of refrigerant) of the refrigeration cycle device in a full cooling operation. FIG. 4 is a diagram showing the operation (flow of refrigerant) in a full heating operation of the refrigeration cycle device. FIG. 4 is a diagram showing the operation (flow of refrigerant) of the refrigeration cycle device in cooling-dominated operation. FIG. 4 is a diagram showing the operation (flow of refrigerant) in a heating-dominated operation of the refrigeration cycle device.

(1) Configuration of the Refrigeration System A refrigeration cycle device 1 shown in Figs. 1 and 2 is used for cooling and heating the inside of a building or the like by performing a vapor compression refrigeration cycle operation.

The refrigeration cycle device 1 has a first circuit (primary side circuit) 5a, a second circuit (secondary side circuit) 10, and a cascade heat exchanger 35. The refrigeration cycle device 1 of this embodiment has a binary refrigerant circuit consisting of a vapor compression type first circuit 5a and a vapor compression type second circuit 10, and performs a binary refrigeration cycle.

The first circuit 5a circulates a first refrigerant and a first refrigeration oil. The first refrigerant includes, for example, at least one of an HFC refrigerant and an HFO refrigerant. In this embodiment, the first refrigerant is R32. The first refrigeration oil is, for example, polyvinyl ether oil.

The second circuit 10 circulates carbon dioxide refrigerant and a second refrigeration oil. The second refrigeration oil is, for example, incompatible with carbon dioxide. In this embodiment, the second refrigeration oil is polyalkylene glycol oil.

The first circuit 5a and the second circuit 10 are thermally connected via a cascade heat exchanger 35.

The refrigeration cycle device 1 is configured by connecting a first unit 5, a cascade unit 2, and second units 4a, 4b, and 4c to each other via piping. The first unit 5 and the cascade unit 2 are connected by a first connecting pipe 112 and a second connecting pipe 111. The cascade unit 2 and the multiple branch units 6a, 6b, and 6c are connected by three connecting pipes: a third connecting pipe 7, a fourth connecting pipe 8, and a fifth connecting pipe 9. The multiple branch units 6a, 6b, and 6c are connected to the multiple utilization units 3a, 3b, and 3c by first connecting pipes 15a, 15b, and 15c and second connecting pipes 16a, 16b, and 16c.

In this embodiment, there is one first unit 5. In this embodiment, there is one cascade unit 2. In this embodiment, there are three second units 4a, 4b, 4c. In detail, the second units 4a, 4b, 4c include branching units 6a, 6b, 6c and usage units 3a, 3b, 3c. The multiple usage units 3a, 3b, 3c of the second units 4a, 4b, 4c are three units, namely, the first usage unit 3a, the second usage unit 3b, and the third usage unit 3c. The multiple branching units 6a, 6b, 6c of the second units 4a, 4b, 4c are three units, namely, the first branching unit 6a, the second branching unit 6b, and the third branching unit 6c.

The refrigeration cycle device 1 is configured so that each utilization unit 3a, 3b, 3c can individually perform cooling or heating operation, and can recover heat between utilization units by sending refrigerant from a utilization unit performing heating operation to a utilization unit performing cooling operation. Specifically, in this embodiment, heat recovery is performed by performing cooling-dominated operation or heating-dominated operation, which simultaneously performs cooling operation and heating operation. The refrigeration cycle device 1 is also configured to balance the heat load of the cascade unit 2 according to the overall heat load of the multiple utilization units 3a, 3b, 3c, which also takes into account the above-mentioned heat recovery (cooling-dominated operation or heating-dominated operation).

(2) First Circuit The first circuit 5a includes a first compressor 71, a first switching mechanism 72, a first heat exchanger 74, a first expansion valve 76, a first subcooling heat exchanger 103, a first subcooling circuit 104, a first subcooling expansion valve 104a, a second shutoff valve 108, a second expansion valve 102, a cascade heat exchanger 35 shared with the second circuit 10, a first shutoff valve 109, and a first accumulator 105. The first circuit 5a also includes a first flow path 35b of the cascade heat exchanger 35.

The first compressor 71 is a device for compressing the first refrigerant, and is, for example, a volumetric compressor such as a scroll type whose operating capacity can be varied by inverter controlling the compressor motor 71a.

The first accumulator 105 is provided midway through the intake passage that connects the first switching mechanism 72 and the intake side of the first compressor 71.

When the cascade heat exchanger 35 functions as an evaporator of the first refrigerant, the first switching mechanism 72 is in a fourth connection state connecting the suction side of the first compressor 71 and the gas side of the first flow path 35b of the cascade heat exchanger 35 (see the solid line of the first switching mechanism 72 in FIG. 1). When the cascade heat exchanger 35 functions as a radiator of the first refrigerant, the first switching mechanism 72 is in a fifth connection state connecting the discharge side of the first compressor 71 and the gas side of the first flow path 35b of the cascade heat exchanger 35 (see the dashed line of the first switching mechanism 72 in FIG. 1). In this way, the first switching mechanism 72 is a device that can switch the flow path of the refrigerant in the first circuit 5a, and is, for example, a four-way switching valve. And, by changing the switching state of the first switching mechanism 72, it is possible to make the cascade heat exchanger 35 function as an evaporator or a radiator of the first refrigerant.

The cascade heat exchanger 35 is a device for performing heat exchange between the first refrigerant, such as R32, and the carbon dioxide refrigerant without mixing them. The cascade heat exchanger 35 is, for example, a plate-type heat exchanger. The cascade heat exchanger 35 has a second flow path 35a belonging to the second circuit 10 and a first flow path 35b belonging to the first circuit 5a. The second flow path 35a has a gas side connected to the second switching mechanism 22 via the third heat source piping 25, and a liquid side connected to the heat source side expansion valve 36 via the fourth heat source piping 26. The first flow path 35b has a gas side connected to the first compressor 71 via the first refrigerant piping 113, the first connection piping 112, the first shutoff valve 109, and the first switching mechanism 72, and a liquid side connected to the second refrigerant piping 114 in which the second expansion valve 102 is provided.

The first heat exchanger 74 is a device for exchanging heat between the first refrigerant and the outdoor air. In the first heat exchanger 74, the first refrigerant obtains cold or hot heat from the outdoor air, which serves as a heat source. The gas side of the first heat exchanger 74 is connected to a pipe extending from the first switching mechanism 72. The first heat exchanger 74 is, for example, a fin-and-tube type heat exchanger made up of a large number of heat transfer tubes and fins.

The first expansion valve 76 is provided in a pipe extending from the liquid side of the first heat exchanger 74 to the first subcooling heat exchanger 103. The first expansion valve 76 is an electrically operated expansion valve with adjustable opening that adjusts the flow rate of the first refrigerant flowing through the liquid side of the first circuit 5a.

The first subcooling circuit 104 branches off between the first expansion valve 76 and the first subcooling heat exchanger 103, and is connected to a portion of the intake passage between the first switching mechanism 72 and the first accumulator 105. The first subcooling expansion valve 104a is provided upstream of the first subcooling heat exchanger 103 in the first subcooling circuit 104, and is an electrically operated expansion valve with adjustable opening that adjusts the flow rate of the first refrigerant, etc.

The first subcooling heat exchanger 103 is a heat exchanger that exchanges heat between the refrigerant flowing from the first expansion valve 76 toward the second shutoff valve 108 and the refrigerant that has been depressurized in the first subcooling expansion valve 104a in the first subcooling circuit 104.

The first connecting pipe 112 is a pipe that connects the first unit 5 and the cascade unit 2. The second connecting pipe 111 is a pipe that connects the first unit 5 and the cascade unit 2.

The second expansion valve 102 is provided in the second refrigerant pipe 114. The second expansion valve 102 is an electrically operated expansion valve with adjustable opening that adjusts the flow rate of the first refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35, etc.

The first shutoff valve 109 is provided between the first connecting pipe 112 and the first switching mechanism 72. The second shutoff valve 108 is provided between the second connecting pipe 111 and the first subcooling heat exchanger 103.

(3) Second Circuit (3-1) Overview of the Second Circuit The second circuit 10 is configured by connecting a plurality of utilization units 3a, 3b, 3c, a plurality of branch units 6a, 6b, 6c, and a cascade unit 2 to each other. Each utilization unit 3a, 3b, 3c is connected to the corresponding branch unit 6a, 6b, 6c in a one-to-one relationship. Specifically, the utilization unit 3a and the branch unit 6a are connected via a first connection pipe 15a and a second connection pipe 16a, the utilization unit 3b and the branch unit 6b are connected via a first connection pipe 15b and a second connection pipe 16b, and the utilization unit 3c and the branch unit 6c are connected via a first connection pipe 15c and a second connection pipe 16c. In addition, each branch unit 6a, 6b, 6c is connected to the cascade unit 2 via three connection pipes, that is, a third connection pipe 7, a fourth connection pipe 8, and a fifth connection pipe 9. Specifically, a third connection pipe 7, a fourth connection pipe 8, and a fifth connection pipe 9 extending from the cascade unit 2 each branch into a plurality of pipes, which are connected to the respective branch units 6a, 6b, and 6c.

The third connecting pipe 7 carries either a gas-liquid two-phase refrigerant or a liquid refrigerant depending on the operating state. The fourth connecting pipe 8 carries either a gas-liquid refrigerant or a supercritical refrigerant depending on the operating state. The fifth connecting pipe 9 carries either a gas-liquid two-phase refrigerant or a gas-liquid refrigerant depending on the operating state.

The second circuit 10 is configured by interconnecting a heat source circuit 12, branch circuits 14a, 14b, and 14c, and utilization circuits 13a, 13b, and 13c.

(3-2) Heat Source Circuit The heat source circuit 12 mainly includes a second compressor 21, a second switching mechanism 22, a first heat source piping 28, a second heat source piping 29, an intake passage 23, a discharge passage 24, a third heat source piping 25, a fourth heat source piping 26, a fifth heat source piping 27, a cascade heat exchanger 35, a heat source side expansion valve 36, a third shut-off valve 32, a fourth shut-off valve 33, a fifth shut-off valve 31, a second accumulator 30, an oil separator 34, an oil return circuit 40, a second receiver 45, a bypass circuit 46, a bypass expansion valve 46a, a second subcooling heat exchanger 47, a second subcooling circuit 48, a second subcooling expansion valve 48a, an oil return passage 23b, and an oil return valve 23c. In addition, the heat source circuit 12 of the second circuit 10 has a second flow path 35 a of the cascade heat exchanger 35 .

The second compressor 21 is a device for compressing the carbon dioxide refrigerant in the heat source circuit 12 of the second circuit, and is, for example, a scroll type or other volumetric compressor whose operating capacity can be varied by inverter controlling the compressor motor 21a. The second compressor 21 is controlled according to the load during operation so that the greater the load, the greater the operating capacity.

The second switching mechanism 22 is a mechanism capable of switching the connection state of the second circuit 10, in particular the flow path of the refrigerant in the heat source circuit 12. In this embodiment, the second switching mechanism 22 has a discharge side communication section 22x, a suction side communication section 22y, a first switching valve 22a, and a second switching valve 22b. The discharge side communication section 22x is connected to the end of the discharge flow path 24 opposite the second compressor 21 side. The suction side communication section 22y is connected to the end of the suction flow path 23 opposite the second compressor 21 side. The first switching valve 22a and the second switching valve 22b are arranged in parallel with each other between the discharge flow path 24 and the suction flow path 23 of the second compressor 21. The first switching valve 22a is connected to one end of the discharge side communication section 22x and one end of the suction side communication section 22y. The second switching valve 22b is connected to the other end of the discharge side communication part 22x and the other end of the suction side communication part 22y. In this embodiment, the first switching valve 22a and the second switching valve 22b are both configured as four-way switching valves. The first switching valve 22a and the second switching valve 22b each have four connection ports, namely, a first connection port, a second connection port, a third connection port, and a fourth connection port. In the first switching valve 22a and the second switching valve 22b of this embodiment, each fourth port is closed and is a connection port that is not connected to the flow path of the second circuit 10. The first switching valve 22a has a first connection port connected to one end of the discharge side communication part 22x, a second connection port connected to the third heat source pipe 25 extending from the second flow path 35a of the cascade heat exchanger 35, and a third connection port connected to one end of the suction side communication part 22y. The first switching valve 22a switches between a switching state in which the first connection port and the second connection port are connected, and the third connection port and the fourth connection port are connected, and a switching state in which the third connection port and the second connection port are connected, and the first connection port and the fourth connection port are connected. The second switching valve 22b switches between a switching state in which the first connection port and the second connection port are connected, and the third connection port and the fourth connection port are connected, and a switching state in which the third connection port and the second connection port are connected, and the first connection port and the fourth connection port are connected.

When the second switching mechanism 22 is to suppress the carbon dioxide refrigerant discharged from the second compressor 21 from being sent to the fourth connecting pipe 8 while the cascade heat exchanger 35 is functioning as a radiator for the carbon dioxide refrigerant, the second switching mechanism 22 is switched to a first connection state in which the discharge flow path 24 and the third heat source pipe 25 are connected by the first switching valve 22a and the first heat source pipe 28 and the suction flow path 23 are connected by the second switching valve 22b. The first connection state of the second switching mechanism 22 is a connection state that is adopted during full cooling operation, which will be described later. In addition, when the cascade heat exchanger 35 is to function as an evaporator for the carbon dioxide refrigerant, the second switching mechanism 22 is switched to a second connection state in which the discharge flow path 24 and the first heat source pipe 28 are connected by the second switching valve 22b and the third heat source pipe 25 and the suction flow path 23 are connected by the first switching valve 22a. The second connection state of the second switching mechanism 22 is a connection state adopted during full heating operation and heating-dominated operation, which will be described later. In addition, when the carbon dioxide refrigerant discharged from the second compressor 21 is sent to the fourth connection pipe 8 while the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant, the second switching mechanism 22 is switched to a third connection state in which the discharge flow path 24 is connected to the third heat source pipe 25 by the first switching valve 22a and the discharge flow path 24 is connected to the first heat source pipe 28 by the second switching valve 22b. The third connection state of the second switching mechanism 22 is a connection state adopted during cooling-dominated operation, which will be described later.

As described above, the cascade heat exchanger 35 is a device for performing heat exchange between the first refrigerant, such as R32, flowing through the first circuit 5a and the carbon dioxide refrigerant flowing through the second circuit 10 without mixing them. The cascade heat exchanger 35 is shared by the first unit 5 and the cascade unit 2 by having a second flow path 35a through which the carbon dioxide refrigerant of the second circuit 10 flows and a first flow path 35b through which the first refrigerant of the first circuit 5a flows. In this embodiment, the cascade heat exchanger 35 is disposed inside the cascade casing of the cascade unit 2. The gas side of the first flow path 35b of the cascade heat exchanger 35 extends through the first refrigerant piping 113 to the first connection piping 112 outside the cascade casing. The liquid side of the first flow path 35b of the cascade heat exchanger 35 passes through a second refrigerant pipe 114 in which a second expansion valve 102 is provided, and extends to a second connection pipe 111 outside the cascade casing.

The heat source side expansion valve 36 is an electrically operated expansion valve with adjustable opening that is connected to the liquid side of the cascade heat exchanger 35 to adjust the flow rate of the carbon dioxide refrigerant flowing through the cascade heat exchanger 35. The heat source side expansion valve 36 is provided in the fourth heat source piping 26.

The third shut-off valve 32, the fourth shut-off valve 33, and the fifth shut-off valve 31 are valves provided at the connection ports to external equipment and piping (specifically, the connecting piping 7, 8, and 9). Specifically, the third shut-off valve 32 is connected to the fourth connecting piping 8 that is drawn out from the cascade unit 2. The fourth shut-off valve 33 is connected to the fifth connecting piping 9 that is drawn out from the cascade unit 2. The fifth shut-off valve 31 is connected to the third connecting piping 7 that is drawn out from the cascade unit 2.

The first heat source pipe 28 is a refrigerant pipe that connects the third shutoff valve 32 and the second switching mechanism 22. Specifically, the first heat source pipe 28 connects the third shutoff valve 32 and the second connection port of the second switching valve 22b of the second switching mechanism 22.

The intake passage 23 is a passage that connects the second switching mechanism 22 and the intake side of the second compressor 21. Specifically, the intake passage 23 connects the intake side connection part 22y of the second switching mechanism 22 and the intake side of the second compressor 21. A second accumulator 30 is provided in the middle of the intake passage 23.

The intake passage 23 includes an intake pipe 23a. The intake pipe 23a connects the intake side of the second compressor 21 to the second accumulator 30. Here, one end of the intake pipe 23a is connected to the intake side of the second compressor 21, and the other end of the intake pipe 23a is connected to the upper part of the second accumulator 30.

The second heat source pipe 29 is a refrigerant pipe that connects the fourth shutoff valve 33 to the middle of the suction passage 23. In this embodiment, the second heat source pipe 29 is connected to the suction passage 23 at a connection point between the suction side communication part 22y of the second switching mechanism 22 and the second accumulator 30.

The discharge flow path 24 is a refrigerant pipe that connects the discharge side of the second compressor 21 to the second switching mechanism 22. Specifically, the discharge flow path 24 connects the discharge side of the second compressor 21 to the discharge side communication section 22x of the second switching mechanism 22.

The third heat source pipe 25 is a refrigerant pipe that connects the second switching mechanism 22 and the gas side of the cascade heat exchanger 35. Specifically, the third heat source pipe 25 connects the second connection port of the first switching valve 22a of the second switching mechanism 22 and the gas side end of the second flow path 35a in the cascade heat exchanger 35.

The fourth heat source pipe 26 is a refrigerant pipe that connects the liquid side (the side opposite the gas side, the side opposite the side where the second switching mechanism 22 is provided) of the cascade heat exchanger 35 to the second receiver 45. Specifically, the fourth heat source pipe 26 connects the liquid side end (the end opposite the gas side) of the second flow path 35a in the cascade heat exchanger 35 to the second receiver 45.

The second receiver 45 stores the carbon dioxide refrigerant. The second receiver 45 is provided between the liquid side of the cascade heat exchanger 35 and the second heat exchangers 52a, 52b, and 52c. The fourth heat source pipe 26, the fifth heat source pipe 27, and the bypass circuit 46 extend from the second receiver 45.

The bypass circuit 46 connects the second heat exchangers 52a, 52b, 52c and the cascade heat exchanger 35 with the suction pipe 23a, which will be described later. Here, the bypass circuit 46 is a refrigerant pipe that connects the gas phase region, which is the upper region inside the second receiver 45, with the suction passage 23. Specifically, the bypass circuit 46 is connected in the suction passage 23 between the second switching mechanism 22 and the second accumulator 30.

The bypass circuit 46 is provided with a bypass expansion valve 46a. The bypass expansion valve 46a is an electric expansion valve that can adjust the amount of refrigerant guided from the second receiver 45 to the suction side of the second compressor 21 by adjusting the opening degree.

The fifth heat source pipe 27 is a refrigerant pipe that connects the second receiver 45 and the fifth shutoff valve 31.

The second supercooling circuit 48 is a refrigerant pipe that connects a part of the fifth heat source pipe 27 to the intake passage 23. Specifically, the second supercooling circuit 48 is connected to the intake passage 23 between the second switching mechanism 22 and the second accumulator 30. In this embodiment, the second supercooling circuit 48 extends so as to branch off from between the second receiver 45 and the second supercooling heat exchanger 47.

The second supercooling heat exchanger 47 is a heat exchanger that performs heat exchange between refrigerant flowing through a flow path belonging to the fifth heat source piping 27 and refrigerant flowing through a flow path belonging to the second supercooling circuit 48. In this embodiment, it is provided between the fifth heat source piping 27, where the second supercooling circuit 48 branches off, and the fifth shut-off valve 31. The second supercooling expansion valve 48a is provided between the branching point of the second supercooling circuit 48 from the fifth heat source piping 27, and the second supercooling heat exchanger 47. The second supercooling expansion valve 48a supplies decompressed refrigerant to the second supercooling heat exchanger 47, and is an electrically-operated expansion valve with an adjustable opening.

The second accumulator 30 is provided on the suction side of the second compressor 21. The second accumulator 30 is a container that stores carbon dioxide refrigerant and second refrigeration oil. The second accumulator 30 is a gas-liquid separator that separates the inflowing fluid into a liquid phase and a gas phase. Extending from the second accumulator 30 are the suction pipe 23a, the oil return passage 23b described below, and a portion of the suction flow passage 23 that is connected to the second switching mechanism 22.

The second accumulator 30 includes a main body 30a, an inlet 30b, a refrigerant outlet 30c, and an oil outlet 30d.

The main body 30a has a shape that can be sealed. The main body 30a is not particularly limited, but may be, for example, cylindrical or U-shaped.

The inlet 30b allows the mixture of carbon dioxide refrigerant and the second refrigeration oil to flow into the main body 30a. The inlet 30b allows the carbon dioxide refrigerant to flow into the main body 30a from the portion of the intake passage 23 that is connected to the second switching mechanism 22. The inlet 30b is provided at the top of the main body 30a.

The refrigerant outlet 30c discharges the carbon dioxide refrigerant stored in the main body 30a. The refrigerant outlet 30c is provided at the top of the main body 30a. The refrigerant outlet 30c returns the carbon dioxide refrigerant to the suction pipe 23a.

The oil outlet 30d discharges the refrigerant oil stored in the main body 30a. The oil outlet 30d is provided at the bottom of the main body 30a. The oil outlet 30d sends the second refrigerant oil to the oil return passage 23b.

The oil return passage 23b is provided to connect the lower part of the second accumulator 30 to the suction pipe 23a. The oil return passage 23b returns the second refrigeration oil from the lower part of the second accumulator 30 to the suction pipe 23a.

In this embodiment, the "lower part of the second accumulator 30" is the bottom surface of the main body 30a serving as a container. In other words, one end of the oil return passage 23b is connected to the bottom surface of the main body 30a of the second accumulator 30. The bottom surface of the main body 30a may have a structure that is curved downward, a structure that is curved upward, a flat structure, etc.

An oil return valve 23c is provided in the oil return passage 23b. By controlling the oil return valve 23c to an open state, the second refrigeration oil separated in the second accumulator 30 passes through the oil return passage 23b and further through the suction pipe 23a, and is returned to the suction side of the second compressor 21.

As long as the oil return valve 23c has a mechanism for narrowing the oil return passage 23b, it may be a solenoid valve that is controlled to open and close, or it may be an electrically operated valve whose opening degree is adjustable. In this embodiment, the oil return valve 23c is a solenoid valve that opens or closes the oil return passage 23b.

The oil separator 34 is provided midway along the discharge flow path 24. The oil separator 34 is a device for separating the second refrigeration oil discharged from the second compressor 21 along with the carbon dioxide refrigerant from the carbon dioxide refrigerant and returning it to the second compressor 21.

The oil return circuit 40 is provided to connect the oil separator 34 and the intake passage 23. The oil return circuit 40 has an oil return passage 41 that extends from the oil separator 34 to join the intake passage 23 between the second accumulator 30 and the intake side of the second compressor 21. An oil return opening/closing valve 44 is provided in the middle of the oil return passage 41. By controlling the oil return opening/closing valve 44 to an open state, the second refrigeration oil separated in the oil separator 34 passes through the oil return passage 41 and is returned to the intake side of the second compressor 21. Here, in this embodiment, when the second compressor 21 is in an operating state in the second circuit 10, the oil return opening/closing valve 44 repeatedly maintains an open state for a predetermined time and a closed state for a predetermined time, thereby controlling the return amount of the second refrigeration oil through the oil return circuit 40. In this embodiment, the oil return opening/closing valve 44 is an electromagnetic valve that is controlled to open and close, but it may be an electric expansion valve whose opening degree can be adjusted.

The heat source circuit 12 constituting the second circuit 10 further has a sensor that measures at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21. Here, a second suction temperature sensor 88 is provided as a sensor that measures the temperature of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21, and a second suction pressure sensor 37 is provided as a sensor that measures the pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21.

(3-3) Utilization Circuit The utilization circuits 13a, 13b, and 13c will be described below. However, since the utilization circuits 13b and 13c have the same configuration as the utilization circuit 13a, the suffix "b" or "c" is added to the reference numerals indicating each part of the utilization circuit 13a instead of the suffix "a" for the utilization circuits 13b and 13c, and the description of each part will be omitted.

The utilization circuit 13a mainly includes a second heat exchanger 52a, a first utilization pipe 57a, a second utilization pipe 56a, and a utilization side expansion valve 51a.

The second heat exchanger 52a is a device for exchanging heat between the refrigerant and the indoor air, and is, for example, a fin-and-tube type heat exchanger composed of a large number of heat transfer tubes and fins. The multiple second heat exchangers 52a, 52b, and 52c are connected in parallel to the second switching mechanism 22, the intake passage 23, and the cascade heat exchanger 35.

One end of the second utilization pipe 56a is connected to the liquid side (opposite the gas side) of the second heat exchanger 52a of the first utilization unit 3a. The other end of the second utilization pipe 56a is connected to the second connection pipe 16a. The above-mentioned utilization side expansion valve 51a is provided midway along the second utilization pipe 56a.

The utilization side expansion valve 51a is an electrically operated expansion valve with adjustable opening that adjusts the flow rate of the refrigerant flowing through the second heat exchanger 52a. The utilization side expansion valve 51a is provided in the second utilization pipe 56a.

One end of the first utilization pipe 57a is connected to the gas side of the second heat exchanger 52a of the first utilization unit 3a. In this embodiment, the first utilization pipe 57a is connected to the side opposite the utilization side expansion valve 51a of the second heat exchanger 52a. The other end of the first utilization pipe 57a is connected to the first connection pipe 15a.

(3-4) Branch Circuit The branch circuits 14a, 14b, and 14c will be described below. However, since the branch circuits 14b and 14c have the same configuration as the branch circuit 14a, the explanation of each part of the branch circuits 14b and 14c will be omitted by adding the suffix "b" or "c" instead of the suffix "a" to the reference numerals indicating each part of the branch circuit 14a.

The branch circuit 14a mainly includes a junction pipe 62a, a first branch pipe 63a, a second branch pipe 64a, a first control valve 66a, a second control valve 67a, a bypass pipe 69a, a check valve 68a, and a third branch pipe 61a.

One end of the junction pipe 62a is connected to the first connection pipe 15a. The other end of the junction pipe 62a is connected to a first branch pipe 63a and a second branch pipe 64a.

The first branch pipe 63a is connected to the fourth connection pipe 8 on the side opposite the junction pipe 62a. The first branch pipe 63a is provided with a first control valve 66a that can be opened and closed.

The second branch pipe 64a is connected to the fifth connecting pipe 9 on the side opposite the junction pipe 62a. The second branch pipe 64a is provided with a second control valve 67a that can be opened and closed.

The bypass pipe 69a is a refrigerant pipe that connects the portion of the first branch pipe 63a that is closer to the fourth connecting pipe 8 than the first control valve 66a and the portion of the second branch pipe 64a that is closer to the fifth connecting pipe 9 than the second control valve 67a. A check valve 68a is provided midway along this bypass pipe 69a. The check valve 68a only allows refrigerant to flow from the second branch pipe 64a side to the first branch pipe 63a side, and does not allow refrigerant to flow from the first branch pipe 63a side to the second branch pipe 64a side.

The third branch pipe 61a has one end connected to the second connection pipe 16a. The other end of the third branch pipe 61a is connected to the third connection pipe 7.

The first branch unit 6a can function as follows when performing the full cooling operation described below by closing the first control valve 66a and opening the second control valve 67a. The first branch unit 6a sends the refrigerant flowing into the third branch pipe 61a through the third connection pipe 7 to the second connection pipe 16a. The refrigerant flowing through the second usage pipe 56a of the first usage unit 3a through the second connection pipe 16a is sent to the second heat exchanger 52a of the first usage unit 3a through the usage side expansion valve 51a. The refrigerant sent to the second heat exchanger 52a evaporates through heat exchange with the indoor air, and then flows through the first connection pipe 15a via the first usage pipe 57a. The refrigerant that has flowed through the first connection pipe 15a is sent to the junction pipe 62a of the first branch unit 6a. The refrigerant that has flowed through the junction pipe 62a does not flow to the first branch pipe 63a side, but flows to the second branch pipe 64a side. The refrigerant flowing through the second branch pipe 64a passes through the second control valve 67a. A portion of the refrigerant that has passed through the second control valve 67a is sent to the fifth connecting pipe 9. The remainder of the refrigerant that has passed through the second control valve 67a flows to branch off into a bypass pipe 69a provided with a check valve 68a, passes through a portion of the first branch pipe 63a, and is then sent to the fourth connecting pipe 8. This allows the total flow cross-sectional area to be increased when the carbon dioxide refrigerant in a gaseous state evaporated in the second heat exchanger 52a is sent to the second compressor 21, thereby reducing pressure loss.

Furthermore, when performing cooling-dominated operation and heating-dominated operation described below, the first branch unit 6a can function as follows when cooling the room in the first utilization unit 3a by closing the first control valve 66a and opening the second control valve 67a. The first branch unit 6a sends the refrigerant flowing into the third branch pipe 61a through the third communication pipe 7 to the second connection pipe 16a. The refrigerant flowing through the second utilization pipe 56a of the first utilization unit 3a through the second connection pipe 16a is sent to the second heat exchanger 52a of the first utilization unit 3a through the utilization side expansion valve 51a. The refrigerant sent to the second heat exchanger 52a evaporates through heat exchange with the indoor air, and then flows through the first connection pipe 15a via the first utilization pipe 57a. The refrigerant that has flowed through the first connection pipe 15a is sent to the junction pipe 62a of the first branch unit 6a. The refrigerant that flows through the junction pipe 62a flows into the second branch pipe 64a, passes through the second control valve 67a, and is then sent to the fifth connection pipe 9.

Furthermore, when performing the full heating operation described later, the first branch unit 6a can function as follows by closing the second control valve 67a and opening the first control valve 66a. In the first branch unit 6a, the refrigerant flowing into the first branch pipe 63a through the fourth connection pipe 8 passes through the first control valve 66a and is sent to the junction pipe 62a. The refrigerant that has flowed through the junction pipe 62a flows through the first utilization pipe 57a of the utilization unit 3a via the first connection pipe 15a and is sent to the second heat exchanger 52a. Then, the refrigerant sent to the second heat exchanger 52a dissipates heat by heat exchange with the indoor air, and then passes through the utilization side expansion valve 51a provided in the second utilization pipe 56a. The refrigerant that has passed through the second utilization pipe 56a flows through the third branch pipe 61a of the first branch unit 6a via the second connection pipe 16a, and is then sent to the third connection pipe 7.

Furthermore, when performing cooling-dominated operation and heating-dominated operation described below, when the first utilization unit 3a heats the room, the first branch unit 6a can function as follows by closing the second control valve 67a and opening the first control valve 66a. In the first branch unit 6a, the refrigerant flowing into the first branch pipe 63a through the fourth connection pipe 8 passes through the first control valve 66a and is sent to the junction pipe 62a. The refrigerant that has flowed through the junction pipe 62a flows through the first utilization pipe 57a of the utilization unit 3a via the first connection pipe 15a and is sent to the second heat exchanger 52a. The refrigerant sent to the second heat exchanger 52a dissipates heat by heat exchange with the indoor air, and then passes through the utilization side expansion valve 51a provided in the second utilization pipe 56a. The refrigerant that passes through the second utilization pipe 56a flows through the third branch pipe 61a of the first branch unit 6a via the second connection pipe 16a, and is then sent to the third connection pipe 7.

This function is not only possessed by the first branching unit 6a, but also by the second branching unit 6b and the third branching unit 6c. Therefore, the first branching unit 6a, the second branching unit 6b and the third branching unit 6c are capable of individually switching between functioning as a refrigerant evaporator or a refrigerant radiator for each of the second heat exchangers 52a, 52b and 52c.

(4) First Unit The first unit 5 is arranged in a space different from the space in which the second units 4a, 4b, and 4c (specifically, the utilization units 3a, 3b, and 3c and the branching units 6a, 6b, and 6c) are arranged. Here, the first unit 5 is arranged on the rooftop of a building.

The first unit 5 is configured to include a portion of the first circuit 5a described above, a first fan 75, various sensors, and a first control unit 70, all housed within a first casing (not shown).

The first unit 5 has, as part of the first circuit 5a, a first compressor 71, a first switching mechanism 72, a first heat exchanger 74, a first expansion valve 76, a first subcooling heat exchanger 103, a first subcooling circuit 104, a first subcooling expansion valve 104a, a second shutoff valve 108, a first shutoff valve 109, and a first accumulator 105.

The first fan 75 is provided in the first unit 5 and generates an air flow in which the outdoor air is guided to the first heat exchanger 74, where it exchanges heat with the first refrigerant flowing through the first heat exchanger 74, and then discharged outside. The first fan 75 is driven by a first fan motor 75a.

Furthermore, the first unit 5 is provided with various sensors. Specifically, the first unit 5 is provided with an outside air temperature sensor 77, a first discharge pressure sensor 78, a first suction pressure sensor 79, a first suction temperature sensor 81, and a first heat exchange temperature sensor 82. The outside air temperature sensor 77 detects the temperature of the outside air before passing through the first heat exchanger 74. The first discharge pressure sensor 78 detects the pressure of the first refrigerant discharged from the first compressor 71. The first suction pressure sensor 79 detects the pressure of the first refrigerant sucked into the first compressor 71. The first suction temperature sensor 81 detects the temperature of the first refrigerant sucked into the first compressor 71. The first heat exchange temperature sensor 82 detects the temperature of the refrigerant flowing through the first heat exchanger 74.

The first control unit 70 controls the operation of each of the components 71 (71a), 72, 75 (75a), 76, and 104a provided within the first unit 5. The first control unit 70 has a processor and memory, such as a CPU or microcomputer, provided to control the first unit 5, and is capable of exchanging control signals and the like with a remote control (not shown), as well as with the heat source side control unit 20 of the cascade unit 2, the branch unit control units 60a, 60b, and 60c, and the user side control units 50a, 50b, and 50c.

(5) Cascade Unit The cascade unit 2 is arranged in a space different from the space in which the second units 4a, 4b, and 4c (specifically, the utilization units 3a, 3b, and 3c and the branching units 6a, 6b, and 6c) are arranged. Here, the cascade unit 2 is arranged on the rooftop of a building.

The cascade unit 2 is connected to the branch units 6a, 6b, and 6c via the connecting pipes 7, 8, and 9, and constitutes part of the second circuit 10. The cascade unit 2 is also connected to the first unit 5 via the connecting pipes 111 and 112, and constitutes part of the first circuit 5a.

The cascade unit 2 is configured to have the above-mentioned heat source circuit 12, various sensors, the heat source side control unit 20, and the second expansion valve 102, first refrigerant pipe 113, and second refrigerant pipe 114 that constitute part of the first circuit 5a, all housed within a cascade casing (not shown).

The cascade unit 2 is provided with various sensors. Specifically, the cascade unit 2 is provided with the second suction pressure sensor 37, the second discharge pressure sensor 38, the second discharge temperature sensor 39, the second suction temperature sensor 88, the cascade temperature sensor 83, the receiver outlet temperature sensor 84, the bypass circuit temperature sensor 85, the subcooling outlet temperature sensor 86, and the subcooling circuit temperature sensor 87. The second suction pressure sensor 37 detects the pressure of the carbon dioxide refrigerant on the suction side of the second compressor 21. The second discharge pressure sensor 38 detects the pressure of the carbon dioxide refrigerant on the discharge side of the second compressor 21. The second discharge temperature sensor 39 detects the temperature of the carbon dioxide refrigerant on the discharge side of the second compressor 21. The second suction temperature sensor 88 detects the temperature of the carbon dioxide refrigerant on the suction side of the second compressor 21. The cascade temperature sensor 83 detects the temperature of the carbon dioxide refrigerant flowing between the second flow path 35a of the cascade heat exchanger 35 and the heat source side expansion valve 36. The receiver outlet temperature sensor 84 detects the temperature of the carbon dioxide refrigerant flowing between the second receiver 45 and the second subcooling heat exchanger 47. The bypass circuit temperature sensor 85 detects the temperature of the carbon dioxide refrigerant flowing downstream of the bypass expansion valve 46a in the bypass circuit 46. The subcooling outlet temperature sensor 86 detects the temperature of the carbon dioxide refrigerant flowing between the second subcooling heat exchanger 47 and the fifth stop valve 31. The subcooling circuit temperature sensor 87 detects the temperature of the carbon dioxide refrigerant flowing at the outlet of the second subcooling heat exchanger 47 in the second subcooling circuit 48.

The heat source side control unit 20 controls the operation of each of the members 21 (21a), 22, 23c, 36, 44, 46a, 48a, 102 provided inside the cascade casing (not shown) of the cascade unit 2. The heat source side control unit 20 has a processor such as a CPU or a microcomputer and memory provided to control the cascade unit 2, and is capable of exchanging control signals and the like with the first control unit 70 of the first unit 5, the utilization side control units 50a, 50b, 50c of the utilization units 3a, 3b, 3c, and the branch unit control units 60a, 60b, 60c.

In this way, the heat source side control unit 20 can control not only the components constituting the heat source circuit 12 of the second circuit 10, but also the second expansion valve 102 constituting part of the first circuit 5a. Therefore, the heat source side control unit 20 can control the valve opening degree of the second expansion valve 102 itself based on the status of the heat source circuit 12 that it controls, thereby making it possible to bring the status of the heat source circuit 12 closer to a desired status. Specifically, it becomes possible to control the amount of heat that the carbon dioxide refrigerant flowing through the second flow path 35a of the cascade heat exchanger 35 in the heat source circuit 12 receives from the first refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35 or the amount of heat that it gives to the first refrigerant.

(6) Second Unit The second units 4a, 4b, and 4c include utilization units 3a, 3b, and 3c, branching units 6a, 6b, and 6c, first connecting pipes 15a, 15b, and 15c, and second connecting pipes 16a, 16b, and 16c.

(6-1) Utilization Unit The utilization units 3a, 3b, and 3c are installed in a room of a building or the like by being embedded in or suspended from the ceiling, or by being hung on a wall surface of the room.

The utilization units 3a, 3b, and 3c are connected to the cascade unit 2 via connecting pipes 7, 8, and 9.

The utilization units 3a, 3b, and 3c have utilization circuits 13a, 13b, and 13c that form part of the second circuit 10.

The configuration of the usage units 3a, 3b, and 3c will be described below. Note that the second usage unit 3b and the third usage unit 3c have the same configuration as the first usage unit 3a, so only the configuration of the first usage unit 3a will be described here. For the configurations of the second usage unit 3b and the third usage unit 3c, the suffix "b" or "c" will be added instead of the suffix "a" of the reference numerals indicating each part of the first usage unit 3a, and the description of each part will be omitted.

The first usage unit 3a mainly includes the above-mentioned usage circuit 13a, a second fan 53a, a usage-side control unit 50a, and various sensors. The second fan 53a includes a second fan motor 54a.

The second fan 53a draws indoor air into the utilization unit 3a, exchanges heat with the refrigerant flowing through the second heat exchanger 52a, and then generates an air flow that is supplied to the room as supply air. The second fan 53a is driven by a second fan motor 54a.

The utilization unit 3a is provided with a liquid side temperature sensor 58a that detects the temperature of the refrigerant on the liquid side of the second heat exchanger 52a. The utilization unit 3a is also provided with an indoor temperature sensor 55a that detects the indoor temperature, which is the temperature of the air taken in from inside the room before it passes through the second heat exchanger 52a.

The usage-side control unit 50a controls the operation of each of the components 51a, 53a (54a) that make up the usage unit 3a. The usage-side control unit 50a has a processor and memory, such as a CPU or microcomputer, that are provided to control the usage unit 3a, and is capable of exchanging control signals with a remote control (not shown), as well as with the heat source-side control unit 20 of the cascade unit 2, the branch unit control units 60a, 60b, 60c, and the first control unit 70 of the first unit 5.

The second usage unit 3b has a usage circuit 13b, a second fan 53b, a usage side control unit 50b, and a second fan motor 54b. The third usage unit 3c has a usage circuit 13c, a second fan 53c, a usage side control unit 50c, and a second fan motor 54c.

(6-2) Branching Unit The branching units 6a, 6b, and 6c are installed in a space above the ceiling in a room of a building or the like.

The branching units 6a, 6b, and 6c are connected to the utilization units 3a, 3b, and 3c in a one-to-one correspondence. The branching units 6a, 6b, and 6c are connected to the cascade unit 2 via the connection pipes 7, 8, and 9.

Next, the configuration of the branching units 6a, 6b, and 6c will be described. Note that the second branching unit 6b and the third branching unit 6c have the same configuration as the first branching unit 6a, so only the configuration of the first branching unit 6a will be described here. For the configurations of the second branching unit 6b and the third branching unit 6c, the suffix "b" or "c" will be added instead of the suffix "a" of the reference numerals indicating each part of the first branching unit 6a, and the description of each part will be omitted.

The first branching unit 6a mainly includes the above-mentioned branching circuit 14a and a branching unit control unit 60a.

The branching unit control section 60a controls the operation of each of the components 66a, 67a that make up the branching unit 6a. The branching unit control section 60a has a processor, such as a CPU or a microcomputer, and memory that are provided to control the branching unit 6a, and is capable of exchanging control signals with a remote control (not shown), as well as with the heat source side control section 20 of the cascade unit 2, the utilization units 3a, 3b, 3c, and the first control section 70 of the first unit 5.

The second branching unit 6b has a branching circuit 14b and a branching unit control unit 60b. The third branching unit 6c has a branching circuit 14c and a branching unit control unit 60c.

(7) Control Unit (7-1) Overview In the refrigeration cycle apparatus 1, the heat source side control unit 20, the usage side control units 50a, 50b, and 50c, the branching unit control units 60a, 60b, and 60c, and the first control unit 70 are connected to each other via wired or wireless communication to configure a control unit 80. Therefore, this control unit 80 controls the operation of each member 21 (21a), 22, 23c, 36, 44, 46a, 48a, 51a, 51b, 51c, 53a, 53b, 53c (54a, 54b, 54c), 66a, 66b, 66c, 67a, 67b, 67c, 71 (71a), 72, 75 (75a), 76, 104a, etc. based on detection information from various sensors 37, 38, 39, 83, 84, 85, 86, 87, 88, 77, 78, 79, 81, 82, 58a, 58b, 58c, 56a, 56b, 56c, etc. and instruction information received from a remote control or the like not shown.

Here, the control unit 80 switches between an operation in which the carbon dioxide refrigerant is heated by the first refrigerant in the cascade heat exchanger 35 (in this embodiment, full heating operation or heating-dominated operation) and an operation in which the carbon dioxide refrigerant is cooled by the first refrigerant (in this embodiment, full cooling operation or cooling-dominated operation). In this way, to switch between heating and cooling of the carbon dioxide refrigerant by the cascade heat exchanger 35, the heat source side control unit 20 of the control unit 80 controls the second switching mechanism 22. Specifically, the control unit 80 controls the second switching mechanism 22 to switch between a first state in which the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant, and a second state in which the cascade heat exchanger 35 functions as an evaporator for the carbon dioxide refrigerant.

(7-2) Control During Normal Operation Hereinafter, the control by the control unit 80 during normal refrigeration cycle operation (normal operation) of the refrigeration cycle apparatus 1 will be described. The normal refrigeration cycle operation of the refrigeration cycle apparatus 1 includes full cooling operation, full heating operation, cooling-dominated operation, and heating-dominated operation. In this embodiment, the control unit 80 performs the following control during operation in which the carbon dioxide refrigerant is heated by the first refrigerant in the cascade heat exchanger 35. Here, the control unit 80 performs the following control during full heating operation and heating-dominated operation.

The control unit 80 controls the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 is equal to or higher than a predetermined temperature or pressure corresponding to the boundary temperature at which the density of the carbon dioxide refrigerant in the second accumulator 30 is equal to the density of the second refrigeration oil. In this embodiment, the heat source side control unit 20 of the cascade unit 2 sends a command to the first control unit 70 of the first unit 5 to perform the above control.

By performing this control, even when the outside air temperature is low, the density of the second refrigeration oil in the second accumulator 30 located outside the room is prevented from becoming lower than the density of the carbon dioxide refrigerant. In other words, the control unit 80 controls the second refrigeration oil to be located below the carbon dioxide refrigerant in the second accumulator 30. In further other words, the control unit 80 prevents the second refrigeration oil from reversing from below to above the carbon dioxide refrigerant in the second accumulator 30.

The "predetermined temperature or pressure" is a temperature or pressure equal to or higher than the boundary temperature, and preferably exceeds the boundary temperature. The boundary temperature is -15°C when the second refrigeration oil is polyalkylene glycol.

In this embodiment, in order to perform the above control, the control unit 80 sets a target evaporation temperature or target evaporation pressure of the carbon dioxide refrigerant in the cascade heat exchanger 35. In other words, the control unit 80 controls the operation of the first circuit 5a so that the carbon dioxide refrigerant is at or above the target evaporation temperature or target evaporation pressure, so that the density of the carbon dioxide refrigerant and the density of the second refrigeration oil do not invert.

Specifically, the control unit 80 controls the rotation speed of the first compressor 71 of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 is equal to or higher than a predetermined temperature or pressure. In particular, during full heating operation or heating-dominated operation, the control unit 80 sets a target condensing temperature of the first refrigerant in the cascade heat exchanger 35 so that the carbon dioxide refrigerant becomes the target evaporation temperature or target evaporation pressure, and controls the rotation speed of the first compressor 71 so that the target condensing temperature is reached.

In addition, in this embodiment, in order to determine whether the density of the carbon dioxide refrigerant and the density of the second refrigeration oil are inverted, the control unit 80 acquires the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil flowing through the suction pipe 23a from a sensor that measures at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21. Here, the control unit 80 acquires the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil flowing through the suction pipe 23a from at least one of the second suction temperature sensor 88 and the second suction pressure sensor 37. Then, the control unit 80 determines whether the acquired temperature or pressure is equal to or higher than a predetermined temperature or pressure that corresponds to a boundary temperature at which the density of the carbon dioxide refrigerant and the density of the second refrigeration oil in the second accumulator 30 become equal.

In addition, in this embodiment, in order to prevent liquid refrigerant from flowing into the second compressor 21, the control unit 80 changes the opening degree of the oil return valve 23c based on the degree of superheat (discharge superheat) of the carbon dioxide refrigerant discharged from the second compressor 21.

The degree of superheat (discharge superheat) of the carbon dioxide refrigerant discharged from the second compressor 21 is the discharge temperature of the second compressor 21 minus the temperature equivalent to the condensation temperature calculated from the discharge pressure detected by the second discharge pressure sensor 38.

Here, since the oil return valve 23c is an open/close valve, the control unit 80 opens the oil return valve 23c if the discharge superheat of the second circuit 10 is equal to or greater than a predetermined value, and closes the oil return valve 23c if the discharge superheat is less than the predetermined value. The predetermined value is a value at which the carbon dioxide refrigerant is not determined to be wet suction. In detail, the control unit 80 acquires the discharge pressure from the second discharge pressure sensor 38 and calculates the discharge superheat. Then, when the control unit 80 determines that the calculated discharge superheat is smaller than the predetermined value and that there is wet suction, it closes the oil return valve 23c. This makes it possible to prevent the fluid in the second accumulator 30 from flowing into the suction pipe 23a from below the second accumulator 30. On the other hand, when the control unit 80 determines that the calculated discharge superheat is equal to or greater than a predetermined value and that there is no wet suction, it opens the oil return valve 23c. This causes the fluid in the second accumulator 30 to flow into the suction pipe 23a from below the second accumulator 30.

(8) Operation of the Refrigeration Cycle Apparatus Next, the operation of the refrigeration cycle apparatus 1 will be described with reference to FIGS.

The refrigeration cycle operation of the refrigeration cycle device 1 can be mainly divided into full cooling operation, full heating operation, cooling-dominated operation, and heating-dominated operation.

Here, the full cooling operation is a refrigeration cycle operation in which only the utilization units in which the second heat exchangers 52a, 52b, and 52c function as evaporators for the carbon dioxide refrigerant are present, and the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant for the evaporation load of the entire utilization units.

In the full heating operation, there are only utilization units in which the second heat exchangers 52a, 52b, and 52c function as radiators of carbon dioxide refrigerant, and the cascade heat exchanger 35 functions as an evaporator of carbon dioxide refrigerant for the heat radiation load of the entire utilization units, which is a refrigeration cycle operation.

Cooling-dominated operation is an operation that mixes utilization units in which the second heat exchangers 52a, 52b, 52c function as evaporators of carbon dioxide refrigerant, and utilization units in which the second heat exchangers 52a, 52b, 52c function as radiators of the refrigerant. Cooling-dominated operation is a refrigeration cycle operation in which, when the evaporative load is the main component of the heat load of the entire utilization units, the cascade heat exchanger 35 is made to function as a radiator of carbon dioxide refrigerant to process the evaporative load of the entire utilization units.

Heating-dominated operation is an operation that mixes utilization units in which the second heat exchangers 52a, 52b, 52c function as refrigerant evaporators and utilization units in which the second heat exchangers 52a, 52b, 52c function as refrigerant radiators. Heating-dominated operation is a refrigeration cycle operation in which, when the heat radiation load is the main component of the overall heat load of the utilization units, the cascade heat exchanger 35 is made to function as an evaporator for carbon dioxide refrigerant to process the heat radiation load of the entire utilization units.

The operation of the refrigeration cycle device 1, including these refrigeration cycle operations, is performed by the control unit 80.

(8-1) Cooling only operation In cooling only operation, for example, the second heat exchangers 52a, 52b, and 52c of the utilization units 3a, 3b, and 3c all function as evaporators of the refrigerant. In cooling only operation, the cascade heat exchanger 35 functions as a radiator of the carbon dioxide refrigerant. In this cooling only operation, the first circuit 5a and the second circuit 10 of the refrigeration cycle device 1 are configured as shown in Figure 3. Note that the arrows attached to the first circuit 5a and the second circuit 10 in Figure 3 indicate the flow of the refrigerant during cooling only operation.

Specifically, in the first unit 5, the cascade heat exchanger 35 is made to function as an evaporator of the first refrigerant by switching the first switching mechanism 72 to the fourth connection state. The fourth connection state of the first switching mechanism 72 is the connection state shown by the solid line in the first switching mechanism 72 in FIG. 3. As a result, in the first unit 5, the first refrigerant discharged from the first compressor 71 passes through the first switching mechanism 72 and dissipates heat by exchanging heat with the outside air supplied from the first fan 75 in the first heat exchanger 74. The first refrigerant that has dissipated heat in the first heat exchanger 74 passes through the first expansion valve 76 controlled to a fully open state, and a portion of the refrigerant flows toward the second stop valve 108 through the first supercooling heat exchanger 103, and the other portion of the refrigerant branches off and flows into the first supercooling circuit 104. The refrigerant flowing through the first supercooling circuit 104 is decompressed when passing through the first supercooling expansion valve 104a. The refrigerant flowing from the first expansion valve 76 toward the second shutoff valve 108 exchanges heat with the refrigerant decompressed by the first subcooling expansion valve 104a and flowing through the first subcooling circuit 104 in the first subcooling heat exchanger 103, and is cooled until it is in a subcooled state. The first refrigerant in the subcooled state is decompressed when passing through the second expansion valve 102 through the second communication pipe 111. Here, the valve opening degree of the second expansion valve 102 is controlled so that the superheat degree of the first refrigerant sucked into the first compressor 71 satisfies a predetermined condition. The first refrigerant decompressed by the second expansion valve 102 evaporates by heat exchange with the carbon dioxide refrigerant flowing through the second passage 35a when flowing through the first passage 35b of the cascade heat exchanger 35, and flows toward the first communication pipe 112. This first refrigerant passes through the first communication pipe 112 and the first shutoff valve 109, and then reaches the first switching mechanism 72. The refrigerant that passes through the first switching mechanism 72 merges with the refrigerant that flows through the first subcooling circuit 104, and is then sucked into the first compressor 71 via the first accumulator 105.

In the cascade unit 2, the second switching mechanism 22 is switched to the first connection state to cause the cascade heat exchanger 35 to function as a radiator for the carbon dioxide refrigerant. In the first connection state of the second switching mechanism 22, the first switching valve 22a connects the discharge flow path 24 to the third heat source pipe 25, and the second switching valve 22b connects the first heat source pipe 28 to the suction flow path 23. Here, the opening degree of the heat source side expansion valve 36 is adjusted. In the first to third utilization units 3a, 3b, and 3c, the second adjustment valves 67a, 67b, and 67c are controlled to the open state. As a result, all of the second heat exchangers 52a, 52b, and 52c of the utilization units 3a, 3b, and 3c function as refrigerant evaporators. In addition, all of the second heat exchangers 52a, 52b, 52c of the utilization units 3a, 3b, 3c and the suction side of the second compressor 21 of the cascade unit 2 are connected via the first utilization pipes 57a, 57b, 57c, the first connection pipes 15a, 15b, 15c, the junction pipes 62a, 62b, 62c, the second branch pipes 64a, 64b, 64c, the bypass pipes 69a, 69b, 69c, a part of the first branch pipes 63a, 63b, 63c, the fourth connection pipe 8 and the fifth connection pipe 9. In addition, the second supercooling expansion valve 48a is controlled to an opening degree such that the degree of supercooling of the carbon dioxide refrigerant flowing from the outlet of the second supercooling heat exchanger 47 toward the third connection pipe 7 satisfies a predetermined condition. The bypass expansion valve 46a is controlled to a closed state. In the utilization units 3a, 3b, and 3c, the opening degree of the utilization side expansion valves 51a, 51b, and 51c is adjusted.

In such a second circuit 10, the high-pressure carbon dioxide refrigerant compressed and discharged by the second compressor 21 is sent to the second flow path 35a of the cascade heat exchanger 35 through the first switching valve 22a of the second switching mechanism 22. In the cascade heat exchanger 35, the high-pressure carbon dioxide refrigerant flowing through the second flow path 35a dissipates heat, and the first refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35 evaporates. The carbon dioxide refrigerant that has dissipated heat in the cascade heat exchanger 35 passes through the heat source side expansion valve 36, the opening of which is adjusted, and then flows into the second receiver 45. A portion of the carbon dioxide refrigerant flowing out of the second receiver 45 branches off and flows into the second supercooling circuit 48, where it is depressurized in the second supercooling expansion valve 48a, and then merges with the suction flow path 23. In the second subcooling heat exchanger 47, another portion of the refrigerant flowing out of the second receiver 45 is cooled by the refrigerant flowing through the second subcooling circuit 48, and then sent to the third connecting pipe 7 through the fifth shutoff valve 31.

Then, the refrigerant sent to the third connection pipe 7 is branched into three and passes through the third branch pipes 61a, 61b, 61c of the first to third branch units 6a, 6b, 6c. The refrigerant that flows through the second connection pipes 16a, 16b, 16c is then sent to the second utilization pipes 56a, 56b, 56c of the first to third utilization units 3a, 3b, 3c. The refrigerant sent to the second utilization pipes 56a, 56b, 56c is sent to the utilization side expansion valves 51a, 51b, 51c of the utilization units 3a, 3b, 3c.

Then, the carbon dioxide refrigerant that has passed through the utilization side expansion valves 51a, 51b, 51c, the opening of which has been adjusted, exchanges heat with the indoor air supplied by the second fans 53a, 53b, 53c in the second heat exchangers 52a, 52b, 52c. As a result, the carbon dioxide refrigerant flowing through the second heat exchangers 52a, 52b, 52c evaporates and becomes a low-pressure gas refrigerant. The indoor air is cooled and supplied to the room. This cools the indoor space. The low-pressure gas refrigerant that has evaporated in the second heat exchangers 52a, 52b, 52c flows through the first utilization pipes 57a, 57b, 57c, and the first connecting pipes 15a, 15b, 15c, and is then sent to the junction pipes 62a, 62b, 62c of the first to third branch units 6a, 6b, 6c.

Then, the low-pressure gas refrigerant sent to the junction pipes 62a, 62b, 62c flows to the second branch pipes 64a, 64b, 64c. A portion of the carbon dioxide refrigerant that has passed through the second control valves 67a, 67b, 67c in the second branch pipes 64a, 64b, 64c is sent to the fifth connection pipe 9. The remaining refrigerant that has passed through the second control valves 67a, 67b, 67c passes through bypass pipes 69a, 69b, 69c, flows through a portion of the first branch pipes 63a, 63b, 63c, and is then sent to the fourth connection pipe 8.

Then, the low-pressure gas refrigerant sent to the fourth connecting pipe 8 and the fifth connecting pipe 9 is returned to the suction side of the second compressor 21 through the third shutoff valve 32, the fourth shutoff valve 33, the first heat source pipe 28, the second heat source pipe 29, the second switching valve 22b of the second switching mechanism 22, the suction flow path 23, the second accumulator 30 and the suction pipe 23a.

In addition, the second refrigeration oil circulating through the second circuit 10 is returned from the bottom of the second accumulator 30 through the oil return passage 23b, the oil return valve 23c, and the suction pipe 23a to the suction side of the second compressor 21.

This is how full cooling operation works.

(8-2) Full heating operation In the full heating operation, for example, the second heat exchangers 52a, 52b, and 52c of the utilization units 3a, 3b, and 3c all function as radiators of the refrigerant. Also, in the full heating operation, the cascade heat exchanger 35 functions as an evaporator of the carbon dioxide refrigerant. In this full heating operation, the first circuit 5a and the second circuit 10 of the refrigeration cycle apparatus 1 are configured as shown in Figure 4. The arrows attached to the first circuit 5a and the second circuit 10 in Figure 4 indicate the flow of the refrigerant during the full heating operation.

Specifically, in the first unit 5, the first switching mechanism 72 is switched to the fifth connection state to cause the cascade heat exchanger 35 to function as a radiator for the first refrigerant. The fifth connection state of the first switching mechanism 72 is the connection state shown by the dashed line in the first switching mechanism 72 in FIG. 4. As a result, in the first unit 5, the first refrigerant discharged from the first compressor 71 and passing through the first switching mechanism 72 passes further through the first connecting pipe 112 and is sent to the first flow path 35b of the cascade heat exchanger 35. The refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35 is condensed by heat exchange with the carbon dioxide refrigerant flowing through the second flow path 35a. The first refrigerant condensed in the cascade heat exchanger 35 passes through the second expansion valve 102, which is controlled to a fully open state, as it flows through the second refrigerant pipe 114. The refrigerant that has passed through the second expansion valve 102 flows through the second connecting pipe 111, the second closing valve 108, and the first subcooling heat exchanger 103 in that order, and is depressurized in the first expansion valve 76. During full heating operation, the first subcooling expansion valve 104a is controlled to a closed state, so that no refrigerant flows through the first subcooling circuit 104, and no heat exchange is performed in the first subcooling heat exchanger 103. The first expansion valve 76 is controlled, for example, so that the degree of superheat of the first refrigerant sucked into the first compressor 71 satisfies a predetermined condition. The refrigerant that has been depressurized in the first expansion valve 76 evaporates by exchanging heat with outside air supplied from the first fan 75 in the first heat exchanger 74, passes through the first switching mechanism 72 and the first accumulator 105, and is sucked into the first compressor 71.

Furthermore, in the cascade unit 2, the second switching mechanism 22 is switched to the second connection state. This causes the cascade heat exchanger 35 to function as an evaporator for the carbon dioxide refrigerant. In the second connection state of the second switching mechanism 22, the discharge flow path 24 and the first heat source pipe 28 are connected by the second switching valve 22b, and the third heat source pipe 25 and the suction flow path 23 are connected by the first switching valve 22a. Furthermore, the opening degree of the heat source side expansion valve 36 is adjusted. In the first to third branch units 6a, 6b, and 6c, the first adjustment valves 66a, 66b, and 66c are controlled to the open state, and the second adjustment valves 67a, 67b, and 67c are controlled to the closed state. As a result, all of the second heat exchangers 52a, 52b, and 52c of the utilization units 3a, 3b, and 3c function as radiators for the refrigerant. The second heat exchangers 52a, 52b, and 52c of the utilization units 3a, 3b, and 3c are connected to the discharge side of the second compressor 21 of the cascade unit 2 via the discharge flow path 24, the first heat source pipe 28, the fourth connecting pipe 8, the first branch pipes 63a, 63b, and 63c, the junction pipes 62a, 62b, and 62c, the first connecting pipes 15a, 15b, and 15c, and the first utilization pipes 57a, 57b, and 57c. The second subcooling expansion valve 48a and the bypass expansion valve 46a are controlled to a closed state. In the utilization units 3a, 3b, and 3c, the utilization side expansion valves 51a, 51b, and 51c are adjusted in opening degree.

In this second circuit 10, the high-pressure carbon dioxide refrigerant compressed and discharged by the second compressor 21 is sent to the first heat source pipe 28 through the second switching valve 22b of the second switching mechanism 22. The refrigerant sent to the first heat source pipe 28 is sent to the fourth connection pipe 8 through the third shutoff valve 32.

The high-pressure refrigerant sent to the fourth connecting pipe 8 is then branched into three and sent to the first branch pipes 63a, 63b, and 63c of the operating utilization units 3a, 3b, and 3c. The high-pressure carbon dioxide refrigerant sent to the first branch pipes 63a, 63b, and 63c passes through the first control valves 66a, 66b, and 66c, and flows through the junction pipes 62a, 62b, and 62c. The refrigerant then flows through the first connecting pipes 15a, 15b, and 15c and the first utilization pipes 57a, 57b, and 57c, and is sent to the second heat exchangers 52a, 52b, and 52c.

The high-pressure carbon dioxide refrigerant sent to the second heat exchangers 52a, 52b, 52c then exchanges heat with the indoor air supplied by the second fans 53a, 53b, 53c in the second heat exchangers 52a, 52b, 52c. As a result, the carbon dioxide refrigerant flowing through the second heat exchangers 52a, 52b, 52c dissipates heat. The indoor air is heated and supplied to the room. This heats the indoor space. The carbon dioxide refrigerant that dissipates heat in the second heat exchangers 52a, 52b, 52c flows through the second utilization pipes 56a, 56b, 56c and passes through the utilization side expansion valves 51a, 51b, 51c, the opening of which is adjusted. After that, the refrigerant that flows through the second connection pipes 16a, 16b, 16c flows through the third branch pipes 61a, 61b, 61c of each branch unit 6a, 6b, 6c.

The carbon dioxide refrigerant sent to the third branch pipes 61a, 61b, and 61c is then sent to the third connecting pipe 7 where they join together.

Then, the carbon dioxide refrigerant sent to the third connecting pipe 7 is sent to the heat source side expansion valve 36 through the fifth shutoff valve 31. The refrigerant sent to the heat source side expansion valve 36 has its flow rate adjusted in the heat source side expansion valve 36, and is then sent to the cascade heat exchanger 35. In the cascade heat exchanger 35, the carbon dioxide refrigerant flowing through the second flow path 35a evaporates into low-pressure gas refrigerant and is sent to the second switching mechanism 22, and the first refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35 condenses. The low-pressure gas refrigerant sent to the first switching valve 22a of the second switching mechanism 22 is returned to the suction side of the second compressor 21 through the suction flow path 23, the second accumulator 30, and the suction pipe 23a.

In addition, the second refrigeration oil circulating through the second circuit 10 is returned from the bottom of the second accumulator 30 through the oil return passage 23b, the oil return valve 23c, and the suction pipe 23a to the suction side of the second compressor 21.

This is how full heating operation works.

During this heating only operation, the control unit 80 controls the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 is equal to or higher than a predetermined temperature or pressure corresponding to the boundary temperature at which the density of the carbon dioxide refrigerant in the second accumulator 30 is equal to the density of the second refrigeration oil. Here, during heating only operation, the control unit 80 sets a target condensing temperature of the first refrigerant so that the carbon dioxide refrigerant becomes the target evaporation temperature or target evaporation pressure in the cascade heat exchanger 35 to prevent the density of the carbon dioxide refrigerant from becoming greater than the density of the second refrigeration oil, and controls the rotation speed of the first compressor 71 so that the target condensing temperature is reached.

In addition, during full heating operation, at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21 is measured to determine whether the density of the carbon dioxide refrigerant is greater than the density of the second refrigeration oil.

In addition, during full heating operation, the control unit 80 controls the opening degree of the oil return valve 23c based on the discharge superheat degree so that liquid refrigerant does not flow into the second compressor 21.

(8-3) Cooling-Dominated Operation In cooling-dominated operation, for example, the second heat exchangers 52a, 52b of the utilization units 3a, 3b function as refrigerant evaporators, and the second heat exchanger 52c of the utilization unit 3c functions as a refrigerant radiator. In cooling-dominated operation, the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant. In this cooling-dominated operation, the first circuit 5a and the second circuit 10 of the refrigeration cycle device 1 are configured as shown in Fig. 5. The arrows attached to the first circuit 5a and the second circuit 10 in Fig. 5 indicate the flow of the refrigerant during cooling-dominated operation.

Specifically, in the first unit 5, the first switching mechanism 72 is switched to the fourth connection state (the state shown by the solid line of the first switching mechanism 72 in FIG. 5 ) to make the cascade heat exchanger 35 function as an evaporator of the first refrigerant. As a result, in the first unit 5, the first refrigerant discharged from the first compressor 71 passes through the first switching mechanism 72 and is condensed in the first heat exchanger 74 by heat exchange with outside air supplied from the first fan 75. The first refrigerant condensed in the first heat exchanger 74 passes through the first expansion valve 76 controlled to a fully open state, and a portion of the refrigerant flows toward the second stop valve 108 through the first supercooling heat exchanger 103, and the other portion of the refrigerant branches off and flows into the first supercooling circuit 104. The refrigerant flowing through the first supercooling circuit 104 is decompressed when passing through the first supercooling expansion valve 104a. The refrigerant flowing from the first expansion valve 76 toward the second shutoff valve 108 exchanges heat with the refrigerant decompressed by the first subcooling expansion valve 104a and flowing through the first subcooling circuit 104 in the first subcooling heat exchanger 103, and is cooled until it is in a subcooled state. The refrigerant in the subcooled state flows through the second communication pipe 111 and is decompressed by the second expansion valve 102. At this time, the valve opening degree of the second expansion valve 102 is controlled so that the degree of superheat of the refrigerant sucked into the first compressor 71 satisfies a predetermined condition, for example. When the first refrigerant decompressed by the second expansion valve 102 flows through the first flow path 35b of the cascade heat exchanger 35, it evaporates by heat exchange with the carbon dioxide refrigerant flowing through the second flow path 35a, and flows toward the first communication pipe 112. After passing through the first communication pipe 112 and the first shutoff valve 109, the first refrigerant reaches the first switching mechanism 72. The refrigerant that passes through the first switching mechanism 72 merges with the refrigerant that flows through the first subcooling circuit 104, and is then sucked into the first compressor 71 via the first accumulator 105.

In the cascade unit 2, the second switching mechanism 22 is switched to a third connection state in which the discharge flow path 24 and the third heat source pipe 25 are connected by the first switching valve 22a, and the discharge flow path 24 and the first heat source pipe 28 are connected by the second switching valve 22b, so that the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant. In addition, the opening degree of the heat source side expansion valve 36 is adjusted. In the first to third branch units 6a, 6b, and 6c, the first adjustment valve 66c and the second adjustment valves 67a and 67b are controlled to an open state, and the first adjustment valves 66a, 66b, and the second adjustment valve 67c are controlled to a closed state. As a result, the second heat exchangers 52a and 52b of the utilization units 3a and 3b function as refrigerant evaporators, and the second heat exchanger 52c of the utilization unit 3c functions as a refrigerant radiator. In addition, the second heat exchangers 52a and 52b of the utilization units 3a and 3b are connected to the suction side of the second compressor 21 of the cascade unit 2 via the fifth interconnection pipe 9, and the second heat exchanger 52c of the utilization unit 3c is connected to the discharge side of the second compressor 21 of the cascade unit 2 via the fourth interconnection pipe 8. In addition, the second supercooling expansion valve 48a is controlled to open so that the degree of supercooling of the carbon dioxide refrigerant flowing from the outlet of the second supercooling heat exchanger 47 toward the third interconnection pipe 7 satisfies a predetermined condition. The bypass expansion valve 46a is controlled to a closed state. In the utilization units 3a, 3b, and 3c, the utilization side expansion valves 51a, 51b, and 51c are adjusted in opening.

In this second circuit 10, a portion of the high-pressure carbon dioxide refrigerant compressed and discharged by the second compressor 21 is sent to the fourth connection pipe 8 through the second switching valve 22b of the second switching mechanism 22, the first heat source pipe 28, and the third shutoff valve 32, and the remainder is sent to the second flow path 35a of the cascade heat exchanger 35 through the first switching valve 22a of the second switching mechanism 22 and the third heat source pipe 25.

The high-pressure refrigerant sent to the fourth connecting pipe 8 is then sent to the first branch pipe 63c. The high-pressure refrigerant sent to the first branch pipe 63c is sent to the second heat exchanger 52c of the utilization unit 3c through the first control valve 66c and the junction pipe 62c.

The high-pressure refrigerant sent to the second heat exchanger 52c then exchanges heat with the indoor air supplied by the second fan 53c in the second heat exchanger 52c. As a result, the carbon dioxide refrigerant flowing through the second heat exchanger 52c dissipates heat. The indoor air is heated and supplied to the room, and the heating operation of the utilization unit 3c is performed. The carbon dioxide refrigerant that has dissipated heat in the second heat exchanger 52c flows through the second utilization pipe 56c, and the flow rate is adjusted in the utilization side expansion valve 51c. After that, the carbon dioxide refrigerant that has flowed through the second connecting pipe 16c is sent to the third branch pipe 61c of the branch unit 6c.

The carbon dioxide refrigerant sent to the third branch pipe 61c is then sent to the third connecting pipe 7.

The high-pressure refrigerant sent to the second flow path 35a of the cascade heat exchanger 35 dissipates heat by exchanging heat with the first refrigerant flowing through the first flow path 35b in the cascade heat exchanger 35. The carbon dioxide refrigerant that has dissipated heat in the cascade heat exchanger 35 flows into the second receiver 45 after the flow rate is adjusted in the heat source side expansion valve 36. A part of the carbon dioxide refrigerant that flows out of the second receiver 45 branches off and flows into the second supercooling circuit 48, and after being depressurized in the second supercooling expansion valve 48a, it merges with the suction flow path 23. In the second supercooling heat exchanger 47, another part of the refrigerant that has flowed out of the second receiver 45 is cooled by the refrigerant flowing through the second supercooling circuit 48, and is sent to the third connection pipe 7 through the fifth shutoff valve 31 and merges with the refrigerant that has dissipated heat in the second heat exchanger 52c.

Then, the refrigerant that joins in the third connection pipe 7 branches into two and is sent to the third branch pipes 61a, 61b of the branch units 6a, 6b. The refrigerant that flows through the second connection pipes 16a, 16b is then sent to the second utilization pipes 56a, 56b of the first to second utilization units 3a, 3b. The refrigerant that flows through the second utilization pipes 56a, 56b passes through the utilization side expansion valves 51a, 51b of the utilization units 3a, 3b.

Then, the refrigerant that has passed through the utilization side expansion valves 51a, 51b, the opening of which has been adjusted, exchanges heat with the indoor air supplied by the second fans 53a, 53b in the second heat exchangers 52a, 52b. As a result, the refrigerant flowing through the second heat exchangers 52a, 52b evaporates and becomes a low-pressure gas refrigerant. The indoor air is cooled and supplied to the room. This cools the indoor space. The low-pressure gas refrigerant that has evaporated in the second heat exchangers 52a, 52b is sent to the junction pipes 62a, 62b of the first and second branch units 6a, 6b.

The low-pressure gas refrigerant sent to the junction pipes 62a, 62b is then sent to the fifth connecting pipe 9 via the second control valves 67a, 67b and the second branch pipes 64a, 64b, where it is joined.

Then, the low-pressure gas refrigerant sent to the fifth connecting pipe 9 is returned to the suction side of the second compressor 21 via the fourth shutoff valve 33, the second heat source pipe 29, the suction passage 23, the second accumulator 30 and the suction pipe 23a.

In addition, the second refrigeration oil circulating through the second circuit 10 is returned from the bottom of the second accumulator 30 through the oil return passage 23b, the oil return valve 23c, and the suction pipe 23a to the suction side of the second compressor 21.

This is how cooling-dominated operation works.

(8-4) Heating-dominated operation In heating-dominated operation, for example, the second heat exchangers 52a, 52b of the utilization units 3a, 3b function as radiators of the refrigerant, and the second heat exchanger 52c functions as an evaporator of the refrigerant. In heating-dominated operation, the cascade heat exchanger 35 functions as an evaporator of the carbon dioxide refrigerant. In this heating-dominated operation, the first circuit 5a and the second circuit 10 of the refrigeration cycle apparatus 1 are configured as shown in Fig. 6. The arrows attached to the first circuit 5a and the second circuit 10 in Fig. 6 indicate the flow of the refrigerant during heating-dominated operation.

Specifically, in the first unit 5, the first switching mechanism 72 is switched to the fifth connection state to cause the cascade heat exchanger 35 to function as a radiator for the first refrigerant. The fifth connection state of the first switching mechanism 72 is the connection state shown by the dashed line in the first switching mechanism 72 in FIG. 6. As a result, in the first unit 5, the first refrigerant discharged from the first compressor 71, passing through the first switching mechanism 72 and the first shut-off valve 109 passes through the first connecting pipe 112 and is sent to the first flow path 35b of the cascade heat exchanger 35. The refrigerant flowing through the first flow path 35b of the cascade heat exchanger 35 condenses by exchanging heat with the carbon dioxide refrigerant flowing through the second flow path 35a. The first refrigerant condensed in the cascade heat exchanger 35 passes through the second expansion valve 102 controlled to a fully open state, and then flows through the second connecting pipe 111, the second closing valve 108, and the first subcooling heat exchanger 103 in that order, and is decompressed in the first expansion valve 76. During heating-based operation, the first subcooling expansion valve 104a is controlled to a closed state, so that no refrigerant flows through the first subcooling circuit 104, and no heat exchange is performed in the first subcooling heat exchanger 103. The first expansion valve 76 is controlled to have a valve opening such that the degree of superheat of the refrigerant drawn into the first compressor 71 satisfies a predetermined condition. The refrigerant decompressed in the first expansion valve 76 evaporates by exchanging heat with outside air supplied from the first fan 75 in the first heat exchanger 74, passes through the first switching mechanism 72 and the first accumulator 105, and is drawn into the first compressor 71.

In the cascade unit 2, the second switching mechanism 22 is switched to the second connection state. In the second connection state of the second switching mechanism 22, the second switching valve 22b connects the discharge flow path 24 to the first heat source piping 28, and the first switching valve 22a connects the third heat source piping 25 to the suction flow path 23. This allows the cascade heat exchanger 35 to function as an evaporator for carbon dioxide refrigerant. In addition, the opening degree of the heat source side expansion valve 36 is adjusted. In the first to third branch units 6a, 6b, 6c, the first adjustment valves 66a, 66b and the second adjustment valve 67c are controlled to the open state, and the first adjustment valve 66c and the second adjustment valves 67a, 67b are controlled to the closed state. As a result, the second heat exchangers 52a and 52b of the utilization units 3a and 3b function as radiators of the refrigerant, and the second heat exchanger 52c of the utilization unit 3c functions as an evaporator of the refrigerant. The second heat exchanger 52c of the utilization unit 3c and the suction side of the second compressor 21 of the cascade unit 2 are connected via the first utilization pipe 57c, the first connection pipe 15c, the junction pipe 62c, the second branch pipe 64c, and the fifth connection pipe 9. The second heat exchangers 52a and 52b of the utilization units 3a and 3b and the discharge side of the second compressor 21 of the cascade unit 2 are connected via the discharge flow path 24, the first heat source pipe 28, the fourth connection pipe 8, the first branch pipes 63a and 63b, the junction pipes 62a and 62b, the first connection pipes 15a and 15b, and the first utilization pipes 57a and 57b. In addition, the second subcooling expansion valve 48a and the bypass expansion valve 46a are controlled to a closed state. In the utilization units 3a, 3b, and 3c, the utilization side expansion valves 51a, 51b, and 51c have their openings adjusted.

In this second circuit 10, the high-pressure carbon dioxide refrigerant compressed and discharged by the second compressor 21 is sent to the fourth connection pipe 8 through the second switching valve 22b of the second switching mechanism 22, the first heat source pipe 28, and the third shutoff valve 32.

The high-pressure refrigerant sent to the fourth connection pipe 8 is then branched into two and sent to the first branch pipes 63a and 63b of the first branch unit 6a and the second branch unit 6b, which are connected to the first and second usage units 3a and 3b, respectively, which are the usage units in operation. The high-pressure refrigerant sent to the first branch pipes 63a and 63b is sent to the second heat exchangers 52a and 52b of the first and second usage units 3a and 3b through the first control valves 66a and 66b, the junction pipes 62a and 62b, and the first connection pipes 15a and 15b.

The high-pressure carbon dioxide refrigerant sent to the second heat exchangers 52a, 52b then exchanges heat with the indoor air supplied by the second fans 53a, 53b in the second heat exchangers 52a, 52b. As a result, the refrigerant flowing through the second heat exchangers 52a, 52b dissipates heat. The indoor air is heated and supplied to the room. This heats the indoor space. The refrigerant that has dissipated heat in the second heat exchangers 52a, 52b flows through the second utilization pipes 56a, 56b and passes through the utilization side expansion valves 51a, 51b, the opening of which is adjusted. The refrigerant that has flowed through the second connection pipes 16a, 16b is then sent to the third connection pipe 7 via the third branch pipes 61a, 61b of the branch units 6a, 6b.

Then, part of the refrigerant sent to the third connecting pipe 7 is sent to the third branch pipe 61c of the branch unit 6c, and the remainder is sent to the heat source side expansion valve 36 through the fifth shutoff valve 31.

Then, the refrigerant sent to the third branch pipe 61c flows through the second utilization pipe 56c of the utilization unit 3c via the second connection pipe 16c, and is sent to the utilization side expansion valve 51c.

Then, the refrigerant that has passed through the utilization side expansion valve 51c, the opening of which has been adjusted, exchanges heat with the indoor air supplied by the second fan 53c in the second heat exchanger 52c. As a result, the refrigerant flowing through the second heat exchanger 52c evaporates and becomes a low-pressure gas refrigerant. The indoor air is cooled and supplied to the room. This cools the indoor space. The low-pressure gas refrigerant that has evaporated in the second heat exchanger 52c passes through the first utilization pipe 57c and the first connecting pipe 15c, and is sent to the junction pipe 62c.

The low-pressure gas refrigerant sent to the junction pipe 62c is then sent to the fifth connection pipe 9 via the second control valve 67c and the second branch pipe 64c.

Then, the low-pressure gas refrigerant sent to the fifth connecting pipe 9 is returned to the suction side of the second compressor 21 via the fourth shutoff valve 33, the second heat source pipe 29, the suction passage 23, the second accumulator 30 and the suction pipe 23a.

The carbon dioxide refrigerant sent to the heat source side expansion valve 36 passes through the heat source side expansion valve 36, the opening of which is adjusted, and then exchanges heat with the first refrigerant flowing through the first flow path 35b in the second flow path 35a of the cascade heat exchanger 35. As a result, the refrigerant flowing through the second flow path 35a of the cascade heat exchanger 35 evaporates and becomes a low-pressure gas refrigerant, which is sent to the first switching valve 22a of the second switching mechanism 22. The low-pressure gas refrigerant sent to the first switching valve 22a of the second switching mechanism 22 merges with the low-pressure gas refrigerant evaporated in the second heat exchanger 52c in the suction flow path 23. The merged refrigerant is returned to the suction side of the second compressor 21 via the second accumulator 30 and the suction piping 23a.

This is how heating-dominated operation works.

During this heating-dominated operation, the control unit 80 controls the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 is equal to or higher than a predetermined temperature or pressure corresponding to the boundary temperature at which the density of the carbon dioxide refrigerant in the second accumulator 30 is equal to the density of the second refrigeration oil. Here, during heating-dominated operation, the control unit 80 sets a target condensing temperature of the first refrigerant so that the carbon dioxide refrigerant becomes the target evaporation temperature or target evaporation pressure in the cascade heat exchanger 35, in order to prevent the density of the carbon dioxide refrigerant from becoming greater than the density of the second refrigeration oil, and controls the rotation speed of the first compressor 71 so that the target condensing temperature is reached.

In addition, during heating-dominated operation, at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21 is measured to determine whether the density of the carbon dioxide refrigerant is greater than the density of the second refrigeration oil.

In addition, during heating-dominated operation, the control unit 80 controls the opening degree of the oil return valve 23c based on the discharge superheat degree so that liquid refrigerant does not flow into the second compressor 21.

(9) Features (9-1)
The refrigeration cycle device 1 of this embodiment includes a first circuit 5a, a second circuit 10, a cascade heat exchanger 35, and a control unit 80. The first circuit 5a circulates a first refrigerant. The second circuit 10 circulates a carbon dioxide refrigerant and a refrigerating machine oil (a second refrigerating machine oil in this embodiment). The cascade heat exchanger 35 heats the carbon dioxide refrigerant with the first refrigerant. The second circuit 10 has a second compressor 21 and a container (a second accumulator 30 in this embodiment). The second accumulator 30 is provided on the suction side of the second compressor 21. The second accumulator 30 stores the carbon dioxide refrigerant and the second refrigerating machine oil. The control unit 80 controls the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigerating machine oil in the second accumulator 30 is equal to or higher than a predetermined temperature or a predetermined pressure corresponding to a boundary temperature at which the density of the carbon dioxide refrigerant and the density of the second refrigerating machine oil in the second accumulator 30 are equal to each other.

In the refrigeration cycle device 1 of this embodiment, the carbon dioxide refrigerant is not heat-exchanged with outside air, the temperature of which naturally changes due to weather changes, but is heat-exchanged with the first refrigerant of the first circuit 5a in the cascade heat exchanger 35. When the carbon dioxide refrigerant of the second circuit 10 is heated in the cascade heat exchanger 35, the control unit 80 controls the operation of the first circuit 5a, so that even when the outside air temperature is low, the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 can be made equal to or higher than a predetermined temperature or pressure corresponding to the boundary temperature at which the density of the carbon dioxide refrigerant and the density of the second refrigeration oil in the second accumulator 30 become equal. Therefore, it is possible to prevent the density of the second refrigeration oil from becoming smaller than the density of the refrigerant in the second accumulator 30.

In this way, the refrigeration cycle device 1 of this embodiment can prevent the density of the second refrigeration oil from becoming lower than the density of the refrigerant in the second accumulator 30 even when the outside air temperature is low, so the second refrigeration oil can be returned to the second compressor 21 during full heating operation and heating-dominated operation. Therefore, the refrigeration cycle device 1 of this embodiment can also be installed in places where the outside air temperature is very low, for example, below -10°C.

(9-2)
The refrigeration cycle apparatus 1 of the present embodiment is the refrigeration cycle apparatus 1 of the above (9-1), and the first circuit 5a has a first compressor 71. The control unit 80 controls the rotation speed of the first compressor 71 of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 becomes equal to or higher than a predetermined temperature or pressure.

Here, the control unit 80 controls the rotation speed of the first compressor 71 of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 is equal to or higher than a predetermined temperature or pressure. This makes it easy to prevent the density of the second refrigeration oil from becoming lower than the density of the refrigerant in the second accumulator 30.

(9-3)
The refrigeration cycle apparatus 1 of this embodiment is the refrigeration cycle apparatus 1 of (9-1) or (9-2) described above, and the control unit 80 switches between an operation in which the carbon dioxide refrigerant is heated by the first refrigerant and an operation in which the carbon dioxide refrigerant is cooled by the first refrigerant in the cascade heat exchanger 35.

Here, the cascade heat exchanger 35 is configured to be switchable between heating and cooling of the carbon dioxide refrigerant. Therefore, the refrigeration cycle device 1 is capable of heating operation (in this embodiment, full heating operation and heating-dominated operation) and cooling operation (in this embodiment, full cooling operation and cooling-dominated operation).

In this embodiment, the refrigeration cycle device 1 further includes a second switching mechanism 22 for switching between heating and cooling the carbon dioxide refrigerant in the cascade heat exchanger 35. The second switching mechanism 22 switches between a state in which the cascade heat exchanger 35 functions as a radiator for the carbon dioxide refrigerant and a state in which the cascade heat exchanger 35 functions as an evaporator for the carbon dioxide refrigerant.

(9-4)
The refrigeration cycle apparatus 1 of this embodiment is the refrigeration cycle apparatus 1 of any one of the above (9-1) to (9-3), and the second circuit 10 further has a sensor (in this embodiment, at least one of the second suction pressure sensor 37 and the second suction temperature sensor 88). The second suction pressure sensor 37 and the second suction temperature sensor 88 measure at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigerating machine oil on the suction side of the second compressor 21.

Here, at least one of the temperature and pressure of the carbon dioxide refrigerant and the second refrigeration oil on the suction side of the second compressor 21 can be measured. Therefore, the density of the second refrigeration oil sucked into the second compressor 21 can be grasped.

(9-5)
The refrigeration cycle apparatus 1 of this embodiment is any one of the refrigeration cycle apparatuses 1 of (9-1) to (9-4) above, and the second circuit 10 further includes a suction pipe 23a, an oil return passage 23b, and a valve (oil return valve 23c in this embodiment). The suction pipe 23a connects the suction side of the second compressor 21 to the second accumulator 30. The oil return passage 23b returns the second refrigeration oil from the lower part of the second accumulator 30 to the suction pipe 23a. The oil return valve 23c is provided in the oil return passage 23b. The control unit 80 changes the opening degree of the oil return valve 23c based on the degree of superheat of the carbon dioxide refrigerant discharged from the second compressor 21.

Here, when the control unit 80 determines that the degree of superheat (discharge superheat) of the carbon dioxide refrigerant discharged from the second compressor 21 is low and wet suction is occurring, it can reduce the opening of the oil return valve 23c in the oil return passage 23b. This makes it possible to prevent liquid-phase carbon dioxide refrigerant from flowing into the second compressor 21.

In order to further prevent liquid-phase carbon dioxide refrigerant from flowing into the second compressor 21, it is preferable that the control unit 80 control the oil return valve 23c in the oil return passage 23b to be fully closed when it determines that wet suction is occurring.

(9-6)
The refrigeration cycle apparatus 1 of this embodiment is any one of the refrigeration cycle apparatuses 1 of (9-1) to (9-5) above, and the first refrigerant is R32.

Here, by circulating R32 in the first circuit 5a, heat exchange with the carbon dioxide refrigerant can be efficiently performed in the cascade heat exchanger 35.

(10) Modifications (10-1) Modification 1
In the above embodiment, the control unit 80 controls the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 becomes equal to or higher than a predetermined temperature or pressure corresponding to a boundary temperature at which the density of the carbon dioxide refrigerant in the second accumulator 30 becomes equal to the density of the second refrigeration oil, during heating only operation and heating main operation, but is not limited thereto. In the refrigeration cycle device disclosed herein, the control unit 80 may control the operation of the first circuit 5a so that the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 becomes equal to or higher than a predetermined temperature or pressure corresponding to a boundary temperature at which the density of the carbon dioxide refrigerant in the second accumulator 30 becomes equal to the density of the second refrigeration oil. Furthermore, in the refrigeration cycle apparatus of the present disclosure, the control unit 80 may control the operation of the first circuit 5a so that, during at least one of full cooling operation and cooling-dominant operation, the temperature or pressure of the carbon dioxide refrigerant and the second refrigeration oil in the second accumulator 30 becomes equal to or higher than a predetermined temperature or predetermined pressure corresponding to the boundary temperature at which the density of the carbon dioxide refrigerant and the density of the second refrigeration oil in the second accumulator 30 become equal.

(10-2) Modification 2
In the above embodiment, polyalkylene glycol has been described as an example of the second refrigeration oil used in the second circuit 10, but the present disclosure is not limited thereto. The second refrigeration oil of the present disclosure may be incompatible with the carbon dioxide refrigerant, in which it is completely incompatible with the carbon dioxide refrigerant, or may be slightly compatible with the carbon dioxide refrigerant but only to a small extent.

(10-3) Modification 3
In the above embodiment, the oil return valve 23c is provided in the oil return passage 23b, but the present modification does not limit the present invention.

(10-4) Modification 4
In the above embodiment, the first refrigerant used in the first circuit 5a is R32, but is not limited thereto. The first refrigerant used in the first circuit 5a may be, for example, R32, R454C, propane, R1234yf, R1234ze, ammonia, or a refrigerant containing any of them.

(10-5) Modification 5
In the above embodiment, the second circuit 10 has three interconnecting pipes 7, 8, and 9, but is not limited to this. The refrigeration cycle device of this modification has two interconnecting pipes. This modification is applied to, for example, a configuration in which the multiple utilization units 3a, 3b, and 3c cannot individually perform cooling or heating operations, a configuration in which there is only one second unit, and the like.

(10-6) Modification 6
In the above embodiment, the refrigeration cycle apparatus 1 in which one cascade unit 2 is connected to one first unit 5 has been described as an example, but is not limited thereto. In the refrigeration cycle apparatus 1 of this modification, a plurality of cascade units 2 are connected in parallel to one first unit 5.

(10-7) Modification 7
In the above embodiment, the refrigeration cycle apparatus 1 in which the plurality of second units 4a, 4b, and 4c are connected to one cascade unit 2 has been described as an example, but is not limited thereto. In the refrigeration cycle apparatus of this modified example, one second unit is connected to one cascade unit 2.

(10-8) Modification 8
In the above embodiment, the first unit 5 is described as an outdoor unit having the first fan 75 for supplying outdoor air to the first heat exchanger 74 for heat exchange with the first refrigerant, but is not limited to this. In this modification, the first unit does not have the first fan 75, and exchanges heat between the first refrigerant and water as a heat source in the first heat exchanger 74.

　Although the embodiments of the present disclosure have been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the present disclosure as set forth in the claims.

1: Refrigeration cycle device 5a: First circuit 10: Second circuit 21: Second compressor 23a: Suction pipe 23b: Oil return passage 23c: Oil return valve (valve)
30: Second accumulator (container)
35: Cascade heat exchanger 37: Second suction pressure sensor (sensor)
52a, 52b, 52c: Second heat exchanger 71: First compressor 74: First heat exchanger 80: Control unit 88: Second intake temperature sensor (sensor)

Patent No. 5425221

Claims (6)

1. a first circuit (5a) in which a first refrigerant circulates;
   a second circuit (10) through which carbon dioxide refrigerant and refrigeration oil circulate;
   a cascade heat exchanger (35) for heating the carbon dioxide refrigerant with the first refrigerant;
   A control unit (80);
   Equipped with
   The second circuit is
   A second compressor (21);
   a container (30) provided on a suction side of the second compressor and configured to store the carbon dioxide refrigerant and the refrigeration oil;
   having
   The control unit controls the operation of the first circuit so that the temperature or pressure of the carbon dioxide refrigerant and the refrigeration oil in the container is equal to or higher than a predetermined temperature or a predetermined pressure corresponding to a boundary temperature at which a density of the carbon dioxide refrigerant and a density of the refrigeration oil in the container are equal to each other.
   Refrigeration cycle device (1).
2. The first circuit includes a first compressor (71),
   The control unit controls the rotation speed of the first compressor of the first circuit so that the temperature or pressure of the carbon dioxide refrigerant and the refrigeration oil in the container becomes equal to or higher than the predetermined temperature or the predetermined pressure.
   The refrigeration cycle device according to claim 1.
3. The control unit, in the cascade heat exchanger,
   An operation in which the carbon dioxide refrigerant is heated by the first refrigerant;
   an operation of cooling the carbon dioxide refrigerant by the first refrigerant;
   Switch between
   The refrigeration cycle apparatus according to claim 1 or 2.
4. The second circuit further includes a sensor (37, 88) for measuring at least one of a temperature and a pressure of the carbon dioxide refrigerant and the refrigeration oil on the suction side of the second compressor.
   The refrigeration cycle device according to any one of claims 1 to 3.
5. The second circuit is
   a suction pipe (23a) connecting a suction side of the second compressor and the container;
   an oil return passage (23b) for returning the refrigeration oil from a lower portion of the container to the suction pipe;
   a valve (23c) provided in the oil return passage;
   and
   The control unit changes an opening degree of the valve based on a degree of superheat of the carbon dioxide refrigerant discharged from the second compressor.
   The refrigeration cycle device according to any one of claims 1 to 4.
6. The first refrigerant comprises R32, R454C, propane, R1234yf, R1234ze or ammonia;
   The refrigeration cycle device according to any one of claims 1 to 5.

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